In many applications, amplifier design requires a balance between linearity and efficiency. Linear (e.g., class A) amplifiers can provide a linear signal at a significant cost in efficiency. More efficient, nonlinear (e.g., class C) amplifiers are available, but these amplifiers tend to suffer from intermodulation and harmonic distortion. A compromise is found in the Doherty amplifier, which utilizes multiple forms of amplifiers to achieve fairly efficient, low distortion amplification of a signal over a wide range of signal power.
FIG. 1 is a functional block diagram of a Doherty amplifier system 10. The Doherty amplifier system 10 comprises a plurality of amplifiers 12 and 14 connected in parallel as to amplify a signal from a signal source 16. The amplifiers include a linear main amplifier 12, which is always operating, and a nonlinear auxiliary amplifier 14 that operates when the power from the signal source reaches a threshold level (e.g., one-quarter peak power). The auxiliary amplifier 14 is connected to the signal source 16 by a first quarter wave transmission line 20, with its output provided to a load 22. The main amplifier 12 is connected to the load through an impedance-inverting network 24 (e.g., quarter wave transmission line).
When the signal power from the signal source 16 is low, the auxiliary amplifier 14 is disabled by a drive control, and the linear main amplifier 12 provides all of the amplification for the signal. When the power of the signal reaches the threshold level, a drive control 18 activates the auxiliary amplifier 14. The activation of the auxiliary amplifier 14 lowers the overall impedance experienced by the main amplifier 12 and allows the signal power to be split between the two amplifiers. Accordingly, the overall efficiency of the system is increased without significantly impacting the linearity of the amplifier system.